In a large majority of commercial aircrafts, the position of the horizontal stabilizer is controlled by a single ball screw actuator with two load paths for safety, for example, a tie-rod within a ball screw and two nuts on the screw. The screw, tie-rod and both nuts have independent attachments to the stabilizer that provide first and second load paths which preclude a catastrophic loss of holding capability with a single structural failure. This is a key requirement for the FAA certification of any commercial aircraft. The second load path may only be used to control the stabilizer upon a failure of the first load path.
This type of architecture requires periodic manual inspection of the second load path to verify its integrity and to identify potential latent failures. Furthermore, this architecture still has the potential for a single failure mechanism in the complete loss of the ball screw threads, which would result in a loss of control of both nuts and a catastrophic loss of control of the stabilizer.